A new congressional report today expressed increasing alarm on the danger posed by Americans of Somali background who have joined Islamic militants, calling them a "direct threat to the U.S. homeland."

The report said there are 40 Americans believed to be training and fighting with al-Shabaab, the al-Qaeda linked terrorist group trying to take control of Somalia.

ABC News has learned that before he died Osama bin Laden was secretly urging al- Shabaab, to target the United States because he was aware of the Americans of Somali descent who had joined al-Shabaab.

The report by the House Homeland Security Committee has found that at least 40 Somali Americans have trained and fought in Somalia over the last four years, including three U.S. suicide bombers who have carried out attacks against African security forces.

The report notes that 15 Americans and three Canadians have been killed fighting with al Shabaab in Somalia. The report also reveals, "At least 21 or more American Shabaab members overseas remain unaccounted for and pose a direct threat to the U.S. homeland."

Chairman of the House Homeland Security Committee, Rep. Peter King, R-N.Y., said in an interview with ABC News, "These are Americans. These are people who have passports, they know the United States, they understand the United States, they know our weak points, they know where they can probe and thrust."

King held a hearing today focused on the terror group, warning that, "With al-Shabaab's large cadre of American jihadis and unquestionable ties to al-Qaeda, particularly its alliance with AQAP [Al Qaeda in the Arabian Peninsula], we must face the reality that al-Shabaab is a growing threat to our homeland."

The House Homeland Security Committee's investigative report released on Wednesday noted, "Shabaab is cementing operational links with Yemeni-American AQAP leader Anwar al-Aulaqi and his growing terror network…U.S. counterterror experts fear the American Shabaab fighters could be taught by AQAP's expert bombmaker to conduct attacks against the United States."

The Somali terror recruits have come from across the country including California, Minnesota, Illinois, Missouri, New York, New Jersey, Virginia, Alabama, Texas.

The largest group of recruits came from the Minneapolis area starting in 2007. According to a Justice Department trial brief filed in the case of Omer Abdi Mohamed, who pleaded guilty earlier this month to recruiting and facilitating travel of young men to Somalia, "Throughout the fall of 2007, the defendant and his conspirators secretly began to mobilize groups of men to depart for Somalia. The group met at mosques, restaurants, and private restaurants to plan the logistics of the trip."

One of the men who was recruited was Shirwa Ahmed, who died in October 2008 and is believed to be the first U.S. citizen to die in a suicide bombing. The group has also recruited fighters through former Alabama resident Omar Hammami. Hammami, a U.S. citizen, has been viewed as a key recruiter for Al Shabaab previously appearing in propaganda videos using the name Abu Mansour al-Amriki. Hammami has released rap-style songs singing about the Somali-American fighters from Minneapolis who had been killed in the fighting in Somali.

In a March 2009 propaganda video Hammami was seen on camera praising a terrorist who died during an ambush and appeared on camera saying in English, "We need more like him, so if you can encourage more of your children and more of your neighbors and anyone around you to send people like him to this jihad, it would be a great asset for us."

Americans in Somalia a 'Direct Threat' to U.S Security

In a speech last month when the Obama administration released their counterterrorism strategy, John Brennan, Assistant to the President for Homeland Security and Counterterrorism said, "From the territory it controls in Somalia al-Shabab continues to call for strikes against the United States. As a result, we cannot and will not let down our guard."

Tuesday appearing before the Senate Select Committee on Intelligence for his confirmation hearing Matthew Olsen, the nominee to head the National Counterterrorism Center, was asked to list his view of top threats. "I would say that beyond Al-Qaeda's senior leadership in Pakistan, its presence in Yemen, probably the next most significant terrorist threat may emanate from the Al-Qaeda presence in Somalia."

Officials have been concerned about evidence that the Somalia group is forging an alliance with Al Qaeda in the Arabian Peninsula, the group responsible for the failed 2009 Christmas day underwear bomb plot and the plan last fall to blow up U.S. cargo planes.

In April 2011 Ahmed Abdulkadir Warsame, a senior Al Shabaab commander, was captured by U.S. forces with evidence showing cooperation with Yemen on planning, weapons and explosives training.

The U.S. Military has executed strikes in Somalia targeting at Al Shabaab members. There have been conflicting reports about Omar Hammami being killed and in 2009 US special forces killed Saleh Ali Nabhan, a key Al Qaeda operative who was believed to have taken part in the 1998 attacks on U.S. embassies in Kenya and Tanzania. Nabhan was also believed to have been training some of the US recruits at training in Somalia before he was killed.